Design and Development of a Coherent Detection Rayleigh Doppler Lidar System for Use as an Alternative Velocimetry Technique in Wind Tunnels

Date of Award

2020

Degree Name

M.S. in Aerospace Engineering

Department

Department of Mechanical and Aerospace Engineering and Renewable and Clean Energy

Advisor/Chair

Advisor: Sidaard Gunasekaran

Abstract

Velocity measurement inside of a wind tunnel is an extremely useful quantitative data for a multitude of reasons. One major reason is that velocity has a mathematical relationship with dynamic pressure which in turn influences all the aerodynamic forces on the test model. Many devices and methods exist for measuring velocity inside wind tunnels. At the same time, Doppler wind lidar (light detection and ranging) has been used for decades to make air speed measurements outdoors at long ranges. Lidar has been proven effective for many applications, and it has the potential to solve many of the problems faced by current velocimetry techniques inside wind tunnels. Despite this, minimal research has been performed with Doppler wind lidars inside wind tunnels. While multiple commercial systems exist for making air speed measurements at longer ranges, there are currently no widely available commercial devices designed to work well inside wind tunnels. In this research, initial work is described for the design and development of a continuous wave (CW), coherent wind lidar system. The system is for use as an alternative non-intrusive velocimetry method inside wind tunnels relying on the Doppler effect. A scaled down wind lidar designed to operate at much shorter ranges than current commercial wind lidars can be simpler, less expensive, and require less power. A first iteration of the design was constructed for proof of concept testing with a small-scale wind tunnel at low speeds (7.5-9 m/s). Testing showed that the lidar system could take one-dimensional speed measurements of seeded flow that closely matched Pitot static tube data. When not adding tracer particles to the flow, the lidar return signal was not strong enough for the photodetector used to measure the beat frequency. This research is focused on the process for designing the Doppler wind lidar system, constructing the experimental setup, and studying methods for data analysis. Results of testing presented in the paper and lessons learned were used to create a second iteration of the wind lidar design that can be built for future testing. Not all data analysis methods and experiments described herein were successful, but this documentation will be helpful to future researchers for improving the design and continuing to make progress on a much needed device for wind tunnel velocimetry.

Keywords

Aerospace Engineering, Atmospheric Sciences, Atmosphere, Engineering, Optics, Technology, Coherent Detection, Rayleigh Doppler Lidar, Doppler Lidar, Lidar, Velocimetry, Wind Tunnels, Rayleigh Scattering, Mie Scattering, Ladar, Laser Radar, Fourier Transform, Fast Fourier Transform, FFT, DFT, Discrete Fourier Transform

Rights Statement

Copyright © 2020, author

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